Systems for optical transmission of information signals enjoy inherent advantages that have resulted in a proliferation of such systems in recent years. Optical systems employ carrier frequencies very close to visible light (infrared and ultraviolet are also used), so that fairly wide bandwidth modulation techniques may be used without fear of using up the available spectrum. In fact, amplitude modulation (AM) is often the technique of choice due to its ease of implementation with carrier signals in the visible light spectrum. Simply varying the current through an LED (light emitting diode) produces a direct amplitude modulation of the light energy.
In addition, optical systems are relatively immune to interference from external RF (radio frequency) transmissions, as well as the electromagnetic impulse of a nuclear explosion. Consequently, high-speed signaling with excellent signal-to-noise ratio performance results.
Of course, the use of optical systems for data or voice transmissions is generally restricted to solid waveguide media. These waveguides take the form of fiber optic cables, which are produced from synthetic materials having desirable optical qualities (high transmissivity, high index of refraction, etc.). The light energy is effectively trapped by the fiber optic cables and guided over relatively short distances by a network of such cable assemblies.
While this restriction to fiber optics is perfectly acceptable in common computer data networks such as LAN's (Local Area Networks), for transmission over longer distances it is frequently impracticable to rely upon fiber optics (length of cable required, unfavorable terrain features, desire to retain earthquake immunity, etc.). Microwave frequencies, transmitted through the air without fiber optic cables, are reliable for short-haul, point-to-point applications. For transmission of voice and data to communication units in the land mobile environment, lower frequency RF signals are almost universally employed.
Because of this necessary transition from carrier frequencies in the visible light spectrum to lower RF frequencies, and because AM modulation is generally rejected in the lower frequency communication spectrum in favor of FM (frequency modulation), various methods have been devised to convert the AM modulated light energy to an FM modulated RF carrier. FM modulated radio signals are superior to AM modulated radio signals with regard to preserving the integrity of transmitted signal information for two reasons: first, FM systems possess an intrinsic immunity to natural and man-made electromagnetic noise (which is amplitude varying), and, second, the capture effect of FM systems leads to greater communication reliability. The most common technique for converting AM to FM involves demodulating the information contained in the light wave using a phototransistor detector circuit, then applying the detector output to a varactor diode that forms the frequency determining element of a voltage controlled oscillator (VCO) to produce an FM signal.
The use of such conversion circuitry adds complexity and cost to communication systems that employ a combination of fiber optic networks and RF networks. Accordingly, a need arises for a method for simplifying the conversion of AM modulated light energy to FM modulated RF signals.